Stay-bolt of clad metal and method of manufacturing same



April 26, 1938. H. VON FOR STER 1 2,115,200

STAY BOLT 0F GLAD METAL AND METHOD MANUFACTURING SAME Filed March 2:5, 1937 Q Inventor:

Patented Apr. '26, 1938 UNITED STATES STAY-BOLT OF OF MANUFACTURING SAME Hermann von Forster, Frankfort-\on-the-Haln, G rmany, assignor of one-half to Arnold 'lrocs,

e Berlin- Steglitz,

, and one-half to Akticugo- Vereinigte Dentsche Metallwerkc sellschaft, Frankfort-on-the-Main, Germany Appllc'ation March 23, 1937, Serial No. 132,041

In Germany November 26, 1935 *8 Claim.

This invention relates to stay-bolts of clad metal and to a method of manufacturing the same.

In the manufacture. of rigid stay-bolts, which are preferably screwed with their thicker threaded ends into the boiler wall, it has already been suggested to use clad metals in order to prevent corrosion in the water space of the boiler, that is, that the stay-bolts should be made of two parts lying one inside the other, the inner part or core consisting of metal of, high mechanical strength, for example steel, whilst the outer part consists of ductile, corrosion-resistantmetal, for example copper.

' The outer metal layer was either cast around the core in order to obtain an intimate welded union between the two parts or was applied to the core by rolling tubes one on the'othen' In any case, however, the covering metal had to be fairly thick, 1. e. thicker than the depth ofthe screw thread to be cut at the end of the staybolt, in order to avoid cutting into the core metal which was sensitive to corrosion. At the points where the corrosion-resistant metal meets the core metal, an electrolytic corrosion may even occur in water, which in conjunction with the notch effect of the thread would .have a substantially greater destructive action on a stay-bolt with a steel core than if a steel stay-bolt without a copper coating were used.

The necessity of having a thick coating of covering metal entails the employment of a considerable percentage of the more expensive corrosion-resistant metal, for example copper. This latter metal which forms the outer jacket in such stay-bolts must take up a relatively high percentage of the total cross-section. atthe part of greater diameter. Thus, for example, a stay-bolt with a shaft diameter of 20 mm., a bore of 5 mm. and a copper coating only 2 mm. thickness,

. consists of as much as about 40% of copper.-

Moreover in cutting such stay-bolts from a clad metal comprising a relatively thick protective.

' It is also frequently desirable in stay-bolts of clad metal to remove the protective jacket of softer material at the points where the head ends are gripped, in order to enable the heavier 55 compressive stresses to be taken up by the more diameter.

heat-resistant core metal. However when using a staybolt having a relatively thick outer layer particular diiilculties are encountered. It is then either necessary to form by upsetting step-like n heads having a very thick external diameter and 5 thereafter to remove the covering metal, i. e. before cutting the thread, or to machine off the metal Jacket before the head is upset; and a normal blank head must then be. upset from the exposed thin For these purposes very long 10 additional ends are required. Both methods present technical difficulties in the pressing, particularly for stay-bolts of relatively large The present invention aims at obviating the 5' foregoing diillculties'with clad metal stay-bolts and'at the same time effecting a considerable saving in covering metal. I Y To this end. according to the invention, the.

stay-bolt is provided with a covering of corrosionresistant metal, for example copper or a copper alloy, the thicknessof which is less than the thickness of the thread, and the thread is rolled in instead of being cut into, the stay-bolt end, which has been thickened, for example by heat deformation. In this way it is possible to reduce the proportion of the outer, more expensive metal down to from 10% to 20% of the total crosssection of the stay-bolt, without in any way impairing the corrosion-resistanceof the threaded portion. The thin protective coating is deformed, during the rolling-in of .the thread, ,in accord.- ance with the profile of the latter and at the same time undergoes a considerable cold strengthening, whereby, ,as shown by the results of numerous investigations, both the permanent bending strength and the heat strength of the stay-bolt heads are substantially increased.

, In addition the low resistance to plastic deformation, of the outer ductile metal which is a- 40 primary cause of stay-bolts becoming untight, is so greatly increased, that the stress on the thread is" with certainty kept below the elastic limit, is not the case-with a thicker covering Llayer, particularly if the thread be cut into said covering layer and the thiclmess of the protective metal thus constantly changes over the length of the thread.

If it be nevertheless desired to remove the covering metal at the gripped point, in order still further to increase the heat elasticity of the bolt head, the advantage remains that the diameter of the head end blanks need be only slightly greater before the thread is formed than the diameter of the finished thread. In this man- PATENT orrlcs yams g a net the pressing operation is considerably simpliiied and the proportion of wastage is greatly reduced.

The invention will hereinafter be explained in greater detail with reference to the accompanying drawing, in which:-

Figs. 1 and 2 represent longitudinal sections of stay-bolts of known design, having a thick layer of copper, which serve as a comparison in order to clarify .the invention.

Figs. 3, 4 and 5 are longitudinal sections of three embodiments of stay-bolt in accordance with the present invention.

In said drawing like parts are denoted by like reference characters.

In Fig. l 1; denotes the steel core and b, the known thick copper layer of the stay-bolt. a is the additional end required to form the head 0, with the corresponding copper layer 11'. After upsetting the head 0, the copper layer b remains thereon. The manufacture of the stay-bolt is carried out by upsetting a step-like cylindrical head c, machining off the copper jacket b on the largest part of the head, and then cutting the thread.

In this method of operation, the copper coating must be thicker than the depth of the'thread, in order to avoid cutting into the steel core on one end of the head. In addition, the maximum diameter of the stay-bolt blank must be relatively great at. the head, in order that the copper jacket is reliably removed. This necessitates a very long additional portion for the upsetting of the head.

Fig. 2. shows the manufacture of a stay-bolt having a thick copper jacket b only partially present on the threaded end.

The originating material is a bar having a thick copper coating which is previously machined to such an extent that the head 0 produced by upsetting is for the greatest part freed from the copper coating; cf. parts a 17 In this method of manufacture the barend a required for the formation of the head after the copper jacket b has been machined off is very thin and long, nevertheless it is necessary to make from same a blank head that is thicker than the outer diameter of the thread'd to be cut.

In contradistinction to the foregoing forms of construction, stay-bolts in accordance with the present invention are manufactured in the following manner:

Fig. 3 illustrates a stay-bolt in accordance with the invention, which possesses a thin covering layer of corrosion-resistant metal (approximately 1 mm. thick) enclosing the whole of the threaded end. In this embodiment the stay-bolt blank need be upset only to the mean thread diameter, for which purpose a substantially smaller addition a is naturally required. During the rolling of the thread d, the copper jacket b or 27 conforms to the profile of the thread. In this process, despite the thin copper coating, it is impossible for the steel core to be exposed at the head and thus for electrolytic corrosion to take place. At the same time the copper jacket is greatly strengthened at the threaded portion d by virtue of the rolling, and is therefore not so easily deformable even at the temperature at which the stay-bolt is used. I

Fig. 4 illustrates the method of the present invention in cases where it is desired, as in Fig. l, to remove the copper jacket b from a part of the threaded end d. In this case the greatest blank diameter need only be equal to the mean diameter of the thread plus the double thickness of the covering metal, since the thread is rolled in, while in the corresponding form with a thick layer of covering metal (shown in Fig. 1) the maximum diameter of the blank must be equal to the outer diameter of the thread d plus double the thickness of the covering layer b, which in addition, is of course substantially thicker than in the case of Fig. 4.

In Fig. 5 the copper jacket b is lastly machined off, before upsetting the head 0, to such an extent that when the additional end a is upset to the mean diameter of the thread d, the greater part of the thread remains freed from the covering layer, as in Fig. 2. After the head 0 has been formed, the latter is machined only very slightly in order to remove the ridge produced during the upsetting and the surface roughness resulting from the pressing, and then the thread is produced by rolling. y

In the cases ofFigs. 4 and 5 rather more added r'naterial a will naturally be required than in the case of Fig. 3, but substantially less than in the case of Figs. 1 and 2. In addition a bolt as shown in Fig. 5 (in contradistinction to Fig. 2) need merely be upset to the mean diameter of the thread, while there is the additional advantage that in the case of the bolt shown in Fig. 5, the bar end a is thicker after machining off the copper jacket, when using a thin copper jacket b, b and an otherwise equal total dlameter of the added end, than in Fig. 2, and can therefore be made substantially shorter.

The present invention accordingly affords the following advantages:

(l) A saving of material.

(2) The upsetting of the heads, even in the case of thick diameters, is facilitated or in some cases rendered possible for the first time.

(3) The covering of corrosion-resistant metal, as far as it is retained, is also reliably ensured in the threaded portion. 1

(4) The proportion of rejects in the manufac- I ture of stay-bolts, which was hitherto very high, is greatly reduced.

I claim: a

1. Process for manufacturing bi-metal sta bolts having threaded heads from blanks having a coating of corrosion resistant metal of less thickness than the depth of the thread to be produced, which comprises rolling threads on the coated head portions-of said blanks of a depth greater. than the thickness of said coating metal whereby the applied metal completely covers the surface of the screw threads.

'2. Process for manufacturing bi-metal stayboits having threaded heads, comprising provid--.

ing a blank having a thin coating of corrosion resistant metal, and rolling threads on the coated blank of a depth greater than the coating of corrosion resistant metal thereon. A

3. Process for manufacturing stay-bolts comprising providing a blank with a thin coating of corrosion resistant metal, forming heads on said blank, and rolling threads on said heads of a depth greater than the thickness of the coating of corrosion resistant metal thereon.

4. Process of manufacturing stay-bolts having threaded heads comprising providing a steel blank with a coating of corrosion resistant metal thereon, upsetting the ends of said blank to provide head portions, and formng threads on the head portions by rolling, said threads having a depth greater than the thickness of the metal coating.

5'. Process for the manufacture of bi-metal stay-bolts in which a thin coating of corrosion resistant metal covers the body portion and a portion of the threaded heads adjacent the body portion, which comprises providing a blank with a coating of corrosion resistant metal, removing the coating adjacent the ends of said blank, upsetting the ends of said blank'to provide head portions having the portions thereof adjacent the body portion covered with said coating metal, and forming threads on said heads by rolling whereby the turns of said threads adjacent said body portion are completely covered by the corrosion resistant metal.

6. Process for the manufacture of a bi-metal stay-bolt having a threaded head in which a thin coating of corrosion resistant metal covers those threads adjacent the body portion of the bolt only, comprising provdipg a metal blank with a thin coating of corrosion resistant metal, upsetting the end of said blank to provide a head portion having a section adjacent the end of a diameter greater at least by the thickness of the coating metal than the section adjacent the body portion of the bolt, removing metal from said head section of greater diameter to provide a head of uniform diameter, and rolling a thread on said head portion, whereby the turns of said thread adjacent the body portion of said bolt will be provided with a corrosion resistant coating and the remainder of said threads will be comprised entirely of theuncoated metal of said blank.

7. In a clad metal stay-bolt, a body portion and a head portion comprising a steel core and a thin coating of corrosion resistant metal thereon, the coated head portion having threads rolled therein and of a depth greater than the thickness of the coating metal but not penetrating said coating metal. I

8. A clad metal stay-bolt comprising a body portion and a head portion, the body portion and a portion of said head adjacent the body having a thin coating of corrosion resistant metal thereon, said head having threads rolled therein in both the coated and uncoated portions thereof and of a depth greater than the thickness of said coating metal but not penetrating the same,

HERMANN VON FORSTER. 

